High-strength, low-alloy steel for seamless pipes with outstanding weldability and corrosion resistance

ABSTRACT

A high-strength steel and a high-strength, weldable steel pipe. The invention relates to a high-strength steel and to a high-strength, weldable steel pipe. The high-strength, low-alloy steel for seamless steel pipes with outstanding weldability and resistance to stress corrosion cracking with a minimum yield strength of 620 MPa and a tensile strength of at least 690 MPa is characterized by the composition indicated in claim  1.

The invention relates to a high-strength, low-alloy steel for seamless pipes with excellent weldability and resistance against stress corrosion cracking according to claim 1.

Seamless pipes constructed from high-strength steels are used, for example, in pipelines for transporting oil or gas and can be laid onshore as well as offshore.

During the past two years, pipe manufacturers have made significant attempts to, on one hand, satisfy the increased requirements for saving material, for example by reducing the wall thickness without changing the material requirements, and on the other hand, the request for designing pipelines for gas transport operating at higher pressures.

Alloys, which are typically used for seamless pipes in pipelines, are defined for steel grades to 80 ksi (X80) in form of standards, for example API 5L, DNV-OS-F 101 and EN 10208. For high-strength grades above 80 ksi, these standards provide no information with respect to limit values for the alloy elements. In the development of high-strength grades, it must be taken into account that the steel pipes produced from these steels should be weldable and have excellent strength and ductility properties.

Until now, standard steel grades to X80 (R_(p0.2): min. 551 MPa, R_(m): min. 620 MPa) according to API 5L were employed in pipelines; however, there is increased demand for high-strength steels in a strength class to 100 ksi (X100) (R_(p0.2): min. 690 MPa, R_(m): min. 760 MPa).

When these steels are used in pipelines for transporting oil and gas, high demands are made regarding their weldability (e.g., pipe joint welding), the ductility at low temperatures down to −40° C. and resistance against stress corrosion cracking, in particular for gas pipelines transporting gas loaded with H₂S (acid gas).

Steel grades to 100 ksi (X100) or even 120 ksi (X120) are generally known for use with welded steel pipes produced with the UOE process.

The required properties of these steels are not attained by increasing the alloy charge, but combining the lowest possible alloy content with thermo-mechanical rolling of the sheet metal to be formed into a pipe.

However, this production method cannot at all or only in a limited way be applied to hot-rolled seamless pipes, because the specific temperature profile during hot-processing of seamless pipes does not allow the required decrease in the transformation temperature which then would make it possible to apply known concepts for thermo-mechanical treatments.

The required properties for hot-rolled, seamless pipes must be attained by a subsequent tempering treatment, in particular by adapting an alloying process and by deliberately adjusting a fine-grain structure meeting the requirements.

The required increase in strength while maintaining adequate ductility of hot-processed seamless pipes for the aforedescribed applications requires the development of new alloying concepts. In particular, adequate ductility and adequate acid gas resistance accompanied by a good weldability is difficult to attain with the conventional alloying processes in a yield strength range above 500 MPa.

A known mechanism responsible for increasing both the strength and the ductility is a reduction in the grain size. This can be achieved, among others, by alloying additional nickel and molybdenum and the associated decrease in the transition temperature.

Molybdenum also improves hardness retention at higher tempering temperatures and through-hardening. However, from a certain alloy concentration on, addition of nickel significantly degrades the surface quality of the hot-rolled pipes.

Increasing the strength with a significant increase in the carbon content causes deterioration in the ductility and a significant increase of the carbon equivalent.

For this reason, such alloy addition must be accompanied by measures that increase the ductility. The carbon equivalent has frequently proven to be a challenging task which significantly limits the selection of the analysis.

Micro-alloying elements, such as titanium, niobium and vanadium, are additionally employed to increase the strength. Titanium already partially precipitates at high temperatures in the liquid phase as very coarse titanium nitride. Niobium forms niobium carbide precipitates at low temperatures. With further decreasing temperature, vanadium accumulates additionally in form of carbonitrites, i.e., precipitation of VC-particles can be anticipated.

Exceedingly coarse precipitates of these micro-alloying elements frequently negatively affect the ductility and the acid gas resistance. Accordingly, the concentration of these alloying elements must not be too high. In addition, the concentration of carbon and nitrogen required for the formation of the precipitates must be taken into account.

WO 2007/017161 discloses a high-strength low-alloy steel for hot-rolled seamless steel pipes which satisfies the requirements of X100 according to API 5L for welded pipes. This conventional steel alloy has an alloying model with C: 0.03-0.13%, Mn: 0.90-1.80%, Si=0.40%, P=0.020%, S=0.005%, Ni: 0.10-1.00%, Cr: 0.20-1.20%. Mo: 0.15-0.80%, Ca=0.40%, V=0.10%, Nb=0.040%, Ti=0.020%, N=0.011%, and a mixed structure comprised of banite and martensite.

Although this conventional steel has after tempering the mechanical properties and weldability required for X100 (100 ksi), no information is provided relating to possible stress corrosion cracking when used in a gas pipeline for transporting gas loaded with H₂S (acid gas). However, the possible formation of chromium carbides in the conventional steel may adversely affect the acid gas resistance.

Moreover, the Ni-concentrations in conventional steel are very high and can reach 1%, which may cause scaling of the surface during hot-rolling of the pipes, for example during hot pilgering, continuous pipe rolling processes, etc., and may severely degrade the surface quality of the pipes and require costly machining of the surface.

The requirements for pipeline pipes for the aforementioned applications can be summarized as follows:

-   Yield strength R_(p0.2) min: 620 MPa (90ksi) and 690 MPa (100 ksi),     respectively -   Tensile strength Rm min: 690 MPa (90 ksi) and 760 MPa (100 ksi),     respectively -   Notched impact strength A_(v) (longitudinal): 90 J at −40° C. -   Guaranteed general weldability -   Low or limited Ni concentration -   Corrosion resistance even when transporting gas loaded with H₂S     (acid gas)

It is an object of the invention to provide a low-cost, low-alloy steel for producing high-strength, weldable seamless steel pipes, which reliably meets the aforementioned requirements with respect to yield strength, tensile strength and notched impact strength and, in addition, has generally good weldability and adequate corrosion resistance when used with acid gas, and which also has a flawless surface after hot-rolling.

The object is solved with the features of claim 1. Advantageous embodiments are recited in the dependent claims.

According to the teaching of the invention, as a low-alloy steel for producing high-strength, weldable hot-rolled seamless steel pipes, a steel with the following chemical composition is proposed:

-   0.030-0.12% C -   max. 0.40% Si -   1.30-2.00% Mn -   max. 0.015% P -   max. 0.005% S -   0.020-0.050% Al -   0.20-0.60% Ni -   0.10-0.40% Cu -   0.20-0.60% Mo -   0.02-0.10% V -   0.02-0.06% Nb -   max. 0.0100% N     remainder iron with melt-related impurities, with optional addition     of Ti and the condition that the sum of the concentration of Ti+Nb+V     has a value from ≧0.04% to ≦0.15% and the ratio Cu/Ni has a value of     <1.

The steel alloy according to the invention improves on the development of the steels according to API 5L, ISO 3183, DNV-OS-F101 and EN 10208 for pipelines.

Experiments performed in the context of the present invention have surprisingly shown that by eliminating Cr and maintaining a predetermined Cu/Ni ratio, the acid gas resistance can be improved for this strength grade compared to a conventional steel alloy, without negatively affecting the mechanical properties (strength and ductility) and weldability.

In addition to nickel, copper also improves the acid gas resistance. Conversely, additionally alloying copper alone negatively affects the hot-workability and damages the grain boundaries. This is compensated by additionally alloying with nickel at a Cu/Ni ratio (Cu/Ni<1) matched to the acid gas resistance.

The combination of the alloying concept according to the invention as a basic process with tempering required after the hot-forming process bring about the acid gas resistance of the developed seamless steel pipe.

The sum of the concentrations of titanium, niobium and vanadium is, on one hand, sufficiently high at a value of ≧0.04% to attain the required increase in strength, but is also sufficiently low with ≦0.15% so as to ensure the required ductility properties and sufficient acid gas resistance.

Depending on the customer requirements, the alloying concept according to the invention can be used to obtain both a steel with the grade 90 ksi (X90) as well as a steel with the grade 100 ksi (X100) while maintaining all requirements typical for the respective grade.

The seamless steel pipes produced from a process melt with the steel alloy according to the invention listed below have excellent strength and ductility values.

-   0.10% C -   0.30% Si -   1.68% Mn -   0.015% P -   0.002% S -   0.026% Al -   0.19% Cu -   0.48% Ni -   0.37% Mo -   0.047% V -   0.042% Nb -   0.003% Ti -   0.006% N     with Cu/Ni=0.40 and Ti+Nb+V=0.092.

Thereafter, the values listed in the following Table are determined. The values are the average values from each of three tensile samples or three notched impact strength samples. The samples were taken as longitudinal samples from heat-treated pipes produced during operation.

Geometry (OD × R₀₅/ A_(v) WD) R_(t0.5) R_(m) R_(m) (at −40° C.) 114.3 × 7.3 mm 809 MPa 842 MPa 0.96 198 J 168.3 × 12.5 mm 804 MPa 835 MPa 0.96 221 J 168.3 × 25.4 mm 697 MPa 768 MPa 0.91 215 J 193.7 × 12.0 mm 807 MPa 839 MPa 0.96 202 J 193.7 × 25.9 mm 696 MPa 774 MPa 0.90 191 J Requirements X90 >620 MPa >690 MPa >90 J Requirements X100 >690 MPa >760 MPa >90 J OD: outside diameter; WD: wall thickness. 

1.-8. (canceled)
 9. A high-strength, low-alloy steel for seamless steel pipes with excellent weldability and resistance against stress corrosion cracking and with a minimum yield strength of 620 MPa and a tensile strength of at least 690 MPa, comprising in mass-%: 0.030-0.12% C, 0.020-0.050% Al, max. 0.40% Si, 1.30-2.00% Mn, max. 0.015% P, max. 0.005% S, 0.20-0.60% Ni, 0.10-0.40% Cu, 0.20-0.60% Mo, 0.02-0.10% V, 0.02-0.06% Nb, max. 0.0100% N, and remainder iron with melt-related impurities, wherein a ratio Cu/Ni has a value of <1.
 10. The steel of claim 9, further comprising Ti, wherein a sum of Ti+Nb+V concentrations has a value from =0.04% to =0.15%.
 11. The steel of claim 10, wherein the Ti concentration is less than or equal to 0.02%.
 12. The steel of claim 9, comprising in mass-% 0.080-0.11% C 0.020-0.050% Al, 0.25-0.35% Si, 1.65-1.90% Mn, max. 0.015% P, max. 0.005% S, 0.45-0.55% Ni, 0.15-0.20% Cu, 0.35-0.55% Mo, 0.04-0.06% V, 0.04-0.05% Nb, max. 0.006% N.
 13. The steel of claim 9, wherein the steel has a minimum yield strength of 690 MPa and a tensile strength of at least 760 MPa.
 14. A high-strength, weldable seamless steel pipe, produced by hot-rolling followed by tempering, with excellent weldability and resistance against stress corrosion cracking and with a minimum yield strength of 620 MPa and a tensile strength of at least 690 MPa, comprising a steel having the following alloy composition in mass-%: 0.030-0.12% C, 0.020-0.050% Al, max. 0.40% Si, 1.30-2.00% Mn, max. 0.015% P, max. 0.005% S, 0.20-0.60% Ni, 0.10-0.40% Cu, 0.20-0.60% Mo, 0.02-0.10% V, 0.02-0.06% Nb, max. 0.0100% N, and remainder iron with melt-related impurities, wherein a ratio Ni/Cu has a value of =1.
 15. The steel pipe of claim 14, wherein the steel further comprises Ti, and wherein a sum of Ti+Nb+V concentrations has a value <0.15.
 16. The steel pipe of claim 14, wherein the steel has a titanium concentration of less than or equal to 0.020%.
 17. The steel pipe of claim 14, wherein the steel has the following alloy composition in mass-%: 0.080-0.11% C 0.020-0.050% Al, 0.25-0.35% Si, 1.65-1.90% Mn, max. 0.015% P, max. 0.005% S, 0.45-0.55% Ni, 0.15-0.20% Cu, 0.35-0.55% Mo, 0.04-0.06% V, 0.04-0.05% Nb, max. 0.006% N.
 18. The steel pipe of claim 14, wherein the steel has a minimum yield strength of 690 MPa and a tensile strength of at least 760 MPa. 